Hyperspectral imaging sensors typically only register one thin line of an image at a time. The image is built up by scanning the sensor across the scene, e.g. using a motorised stage or using the motion of an aircraft to scan across the landscape (push broom scanning).
When incorporated with aircraft, push broom hyperspectral images are gathered using the forward motion of the aircraft to scan an image sensor across the ground in a swath direction, which is typically perpendicular to the forward motion (track direction). A slit and objective lens is used to project an image of a narrow line on the ground through a wavelength dispersive spectrometer. The geometry is arranged so that the images successively projected are spatial in one direction and spectral in the other.
The spatial resolution of aerial push broom hyperspectral images in the swath direction is determined by the characteristics of the lens optics and camera used. The spatial resolution in the track direction is determined by the speed and height of the aircraft. To create the highest quality imagery for subsequent analysis it is normal to match these two resolutions so that the pixels on the hyperspectral images are “square”.
Spectral resolution is principally determined by the extent of the dispersion produced by the spectrometer compared to the size of the sensor in the track direction. Hence to create well resolved images it is preferable to fly as low and as slowly as practically possible.
Military surveillance aircraft cannot generally fly much lower than about 3 km because of the threat from small arms fire and remotely piloted grenades. The stall speed of these types of aircraft is usually not less than about 45 m/s (˜90 knots). In practice this puts a lower limit on the angular rate of scan of about 15 mrad/s (0.86 deg/s). For a frame rate of 50 Hz, this is 0.3 mrad (0.017 deg), or 0.9 m from 3 km, and as such presents a limit to the spatial resolution attainable.